Gasification is well known in the art and it is practiced worldwide with application to solid and heavy liquid fossil fuels, including refinery bottoms. The gasification process converts carbonaceous materials, such as coal, petroleum, biofuel, or biomass with oxygen at high temperature, i.e., greater than 800° C., into synthesis gas, or syngas, steam and electricity. The syngas can be burned directly in internal combustion engines, or it can be separated or used to produce methanol via synthesis, or converted into synthetic fuels via the Fischer-Tropsch process. There are two reactor types used in gasification: refractory and membrane wall reactors. The latter process requires solid particles in the feedstock and therefore is applied to solid fuels only.
Gasification uses partial oxidation to convert any carbon contained in a feedstock into synthesis gas consisting of carbon monoxide (CO) and hydrogen which in turn can be used in the manufacture of various chemicals ranging from fertilizers to liquid fuels or petrochemicals. According to the desired end product, the gasification process unit or block incorporates several technologies.
For refining applications, the main process block is known as the Integrated Gasification Combined Cycle (IGCC), which converts the feedstock into hydrogen, power and steam. FIG. 1 shows the process flow diagram of a conventional IGCC of the prior art. The IGCC is a complex integrated process, consisting of sections, including feed line 101 and feed preparation 102, air separation unit 180 with oxygen feed 103, gasification reactor 104 producing syngas 106, syngas quench and cooling unit 110, with generated steam 112 and cooled syngas 114 passing to water gas shift reactor 120, acid gas removal (AGR) and sulfur recovery unit (SRU) 130 for treatment of shift gas 122 and separation of carbon dioxide 136 and sulfur 138, high hydrogen syngas recovery 132 and/or gas (WGS) turbine feed 134, and a combined cycle package including gas turbine 140 with air feed 142 for producing electricity 144 and a high pressure discharge 146, a heat recovery steam generator (HRSG) 150 receiving steam 116 and boiler feed water 152 and producing steam 154 and boiler feed 156 for delivery to cooling unit 110, and steam turbine 160 for producing electricity 162.
The air separation unit 180 and most of the downstream processes utilize mature technologies with high on-stream reliability factors. However, the gasifier 104 has a relatively limited lifetime that can be as short as from 3 to 18 months, depending upon the characteristics of the feed and the design of the unit.
Three principal types of gasifier technologies are moving bed, fluidized bed and entrained-flow systems. Each of the three types can be used with solid fuels, but only the entrained-flow reactor has been demonstrated to process liquid fuels. In an entrained flow reactor, the fuel and oxygen and steam are injected at the top of the gasifier through a co-annular burner. The gasification usually takes place in a refractory-lined vessel which operates at a pressure of about 40-60 bar and a temperature in the range of from 1300°-1600° C.
For production of liquid fuels and petrochemicals, the key parameter is the H2/CO ratio of the dry syngas. This ratio is usually between 0.85 and 1.2 depending upon the feedstock characteristics. Thus, additional treatment of the syngas is needed to increase this ratio up to 2 for Fischer-Tropsch applications or to convert CO to hydrogen through the water gas shift reaction represented by CO+H2O═CO2+H2. In some cases, part of the syngas is burned together with some off gases in a combined cycle to produce power and steam. The overall efficiency of this process is between 44% and 48%.
The major benefits for a refinery using a heavy residue gasification process are that it provides a source of hydrogen for hydroprocessing to meet the demand for light products; it produces power and steam for refinery use or for export and sale; it can take advantage of efficient power generation technology as compared to conventional technologies that combust heavy residue; and it produces lower pollutant emissions as compared to conventional technologies that combust heavy residues for disposal. Furthermore, the process provides a local solution for heavy residue where produced, thus avoiding off-site transportation or storage; it also provides the potential for disposal of other refinery waste streams, including hazardous materials; and a potential carbon management tool, i.e., a CO2 capture option is provided if required by the local regulatory system.
Gasification technology has a long history of research and development, and several units are already under operation worldwide. For refining applications, it is particularly recommended in some cases where hydrogen is needed for hydroprocessing and natural gas is not available, and the prices of the feed used for gasification are very low. This is usually the case in refineries where full conversion is required to meet the demand of cleaner light products, such as gasoline, jet fuel and diesel transportation fuels.
The gasifier conventionally uses refractory liners to protect the reactor vessel from elevated temperatures that range from 1400° to 1700° C., corrosive slag and thermal cycling. The refractory is subjected to the penetration of corrosive components from the syngas and slag and thus subsequent reactions in which the reactants undergo significant volume changes that result in strength degradation of the refractory materials. The replacement of refractory linings can cost several millions of dollars a year and several weeks of downtime for a given reactor. Up until now, the solution has been the installation of a second or parallel gasifier to provide the necessary capacity, but the undesirable consequence of this duplication is a significant increase in the capital costs associated with the unit operation.
Research has been reported that is directed to means that will increase the useful life of the gasifier refractory material and thus increase the economic competitiveness of the gasification process. This includes new refractory materials and new technologies such as membrane reactors which are expected to have high reliability and high availability compared to that of conventional lined refractory reactors.
Membrane wall gasifier technology uses a cooling screen protected by a layer of refractory material to provide a surface on which the molten slag solidifies and flows downward to the quench zone at the bottom of the reactor. The advantages of the membrane wall reactor include reduced reactor dimensions as compared to other systems and elimination of the need to have a parallel reactor to maintain continuous operation as in the case of refractory wall reactors; the on-stream time for a typical refractory wall reactor is 50%, therefore a parallel unit is required; however, the on-stream time for membrane wall reactors is 90% and there is no need for a second, parallel reactor; and the build-up of a layer of solid and liquid slag provides self-protection to the water-cooled wall sections.
The build-up of a layer of solidified mineral ash slag on the wall acts as an additional protective surface and insulator to minimize or reduce refractory degradation and heat losses through the wall. Thus the water-cooled reactor design avoids what is termed “hot wall” gasifier operation, which requires the construction of thick multiple-layers of expensive refractories which will remain subject to degradation. In the membrane wall reactor, the slag layer is renewed continuously with the deposit of solids on the relatively cool surface. Further advantages include short start-up/shut down times; lower maintenance costs than for the refractory type reactor; and the capability of gasifying feedstocks with high ash content, thereby providing greater flexibility in treating a wider range of coals, petcoke, coal/petcoke blends, biomass co-feed, and liquid feedstocks.
There are two principal types of membrane reactor designs that are adopted for processing of solid feedstocks. One such reactor uses vertical tubes in an up-flow process equipped with several burners for solid fuels, e.g., petcoke. A second solid feedstock reactor uses spiral tubes and down-flow processing for all fuels. For solid fuels, a single burner having a thermal output of about 500 MWt has been developed for commercial use.
In both of these reactors, the flow of pressurized cooling water in the tubes is controlled to cool the refractory and ensure the downward flow of the molten slag. Both systems have demonstrated high utility with solid fuels, but not with liquid fuels.
Waste tire processing for environmental reasons and for the value of the recovered hydrocarbons has been undertaken for many years. In the United States, approximately one waste tire is discarded per capita on an annual basis. The discarded tires numbered approximately 242 million nationwide in the US during 1990, exclusive of retreads. Approximately 100,000 tons of waste tires have been discarded annually in Saudi Arabia. This accounts for a substantial quantity of hydrocarbons since each tire weighs about 20 lb. In addition to their hydrocarbon content, these waste tires increase environmental concerns since they are typically disposed of in landfills or accumulate in piles in tire storage areas. Whole waste tires are difficult to dispose of in landfills; they tend to emit gas, harbor rodents and insects, and will move upward in the landfill over time as other wastes consolidate and subside.
Pyrolysis, liquefaction, and gasification are disposal/recovery technologies that have been applied, or considered for application, to different wastes with varied success. These processes have the potential for recovering usable resources, i.e., energy, chemical feedstocks, steel and fibers from waste tires.
Of these three technologies, pyrolysis has been the most commonly applied. Entrepreneurs and major industrial firms, including tire manufacturers, as well as governmental entities worldwide have invested an estimated $100 million in waste tire decomposition projects.
Several commercial-scale pyrolysis or gasification facilities are currently operating in the United States. Although offering the prospect of substantial financial returns, the tire decomposition projects have not been entirely successful because of operating problems, unsafe and dangerous conditions, lack of an adequate supply of suitable feedstock, poor product quality, lack of adequate environmental controls and high operational costs.
Pyrolysis involves heating organic materials without oxygen to break them down to simpler organic compounds. When organic wastes, e.g., waste tires are the feedstock, products of the process include carbon char, oil and gas. Pyrolysis can convert wood to charcoal and a low-btu gas.
The gasification of organic materials is conducted under operating conditions that are between the complete absence of oxygen and a predetermined or calculated stochiometric volume that is sufficient to complete the oxidation reaction. Gasification involves drying and pyrolyzing a feedstock, and oxidizing the solid char to heat the reaction and produce carbon monoxide (CO) in the exiting product gas stream.
Liquefaction is the thermochemical conversion of an organic solid into a petroleum-like liquid. Liquefaction typically involves the production of a liquid composed of heavy molecular compounds from a pyrolytic gas stream. The liquid has properties that are similar, but not identical, to those of petroleum-based fuels. Liquefaction can be described as the manipulation of the pyrolysis process in order to produce a liquid having characteristics similar to petroleum-based liquids.
It is an object of this invention to provide a process for the disposal of waste tires that is reliable, energy-efficient and environmentally acceptable and that is capable of producing products that can be used as a feedstream for other processes in the same refinery.